Cruiser porpoise
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Cruiser porpoise

  1. #1
    Senior Member namba860's Avatar
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    Default Cruiser porpoise

    I just bought this 77 Centurion 21 cruiser mild 460 Ford Berkeley 12JG. The boat has a porpoise to it unless I run the tabs or diverter down but then the boat runs to wet.
    The pump is set back 4.5 inches AA aluminum impeller Dominator bowl droop snoot 4 deg. wedge upside down place diverter nozzle reducer shoe and ride plate stock finger grate. It has Bennett trim tabs and they are mounted straight across the transom not parallel with the bottom ???? The boat was flipped over and the hook was taken out engine runs 4800 rpm. 50 mph on a water psi speedo please help with the set up on this boat.
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    Senior Member H20MOFO's Avatar
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    Quote Originally Posted by namba860 View Post
    I just bought this 77 Centurion 21 cruiser mild 460 Ford Berkeley 12JG. The boat has a porpoise to it unless I run the tabs or diverter down but then the boat runs to wet.
    The pump is set back 4.5 inches AA aluminum impeller Dominator bowl droop snoot 4 deg. wedge upside down place diverter nozzle reducer shoe and ride plate stock finger grate. It has Bennett trim tabs and they are mounted straight across the transom not parallel with the bottom ???? The boat was flipped over and the hook was taken out engine runs 4800 rpm. 50 mph on a water psi speedo please help with the set up on this boat.
    I wonder if it did it with the hook in the bottom...maybe thats why it was there. Nice lookin boat btw.
    Another Hot Boat refugee

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    Senior Member bp298's Avatar
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    try running the tabs down a little and the diverter up a little. or, find a straight snoot instead of the droop and try that.

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    Senior Member Boat 405's Avatar
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    Most of those 21' daycruiser style boats had hook in them from the factory and would ride fairly well with the hook. More often than not when the hook was removed the boats would porpoise. A few degrees of down trim tabs and a little up nozzle on the diverter should help your situation. Those tabs should be mounted parallel to the bottom of the boat like this picture.

    Last edited by Boat 405; 03-16-2015 at 05:24 PM.
    Boat 405.

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    Senior Member old rigger's Avatar
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    Yeah, those tabs are worthless where they're mounted, like widowmaker pointed out they need to be mounted correctly.

    I never worked at a shop where hook was built into the bottom. That's a salesman line of bull to explain away the hook that ended up there from either having the part pulled too soon from the mold, sitting on an incorrect dolly/trailer or having the intake installed wrong. Howard, Rogers, Tahiti...and more all offered that bottom, none had hook built in. That's not to say some didn't leave with hook at the production shops though.

    As a point of reference a new Spectra 20, same as your Centurion (basically), tested out brand new with a big bloc/berk at 54 mph. Stone stock. The sbc I/O's ran 58-9 mph.
    Last edited by old rigger; 03-16-2015 at 06:29 PM.

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    Senior Member Boat 405's Avatar
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    Agreed with Old Rigger, it wasn't ever planned to have hook in them from the factory, just usually pulled too early from the mold or incorrect bunk trailer setup.
    Boat 405.

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    Senior Member namba860's Avatar
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    This boat looks similar to a Spectra 20 but the bottom looks a little different to me this boat came out of the same mold as a Dimarco

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    Senior Member Futs's Avatar
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    Agree with what was said above about the plates, but my concern is, whoever did the bottom mistake the hook with rocker? Some of the larger boat have rocker built into them to roll over and get on plane quicker. If you don't know what your looking at and lay your straight edge on the bottom and see a gap, people seem to think it has hook in it. I bet they took the rocker out when they did the bottom and that is causing the porpoise. Remount those plates, that will help with the ride and eliminate the porpoise. It will however slow you down a bit.

  11. #9
    Senior Member namba860's Avatar
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    The guy I bought it from was a ski racer and is really familiar with boats and owns a body shop Im almost positive its done right. Bergeron marine told him to mount the plates that way. He tried them the traditional way and claimed the boat bow steered real bad???? This boat had a twin turbo engine in it maybe it just needs more power now to get the boat to take a set??? I agree with moving the plates its just a lot of holes to fill. Maybe I will just take the plates off and leave the cylinders on and try it. I don't see why a jet boat needs trim tabs anyways????

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    Senior Member spectralen's Avatar
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    I hate trim tabs.

  13. #11
    Senior Member old rigger's Avatar
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    Quote Originally Posted by namba860 View Post
    This boat looks similar to a Spectra 20 but the bottom looks a little different to me this boat came out of the same mold as a Dimarco
    it may look like a 1000 different hulls, but it (and those other 1000) had it's origin from a Spectra. Here's a Dimarco for sale right now in Havasu, like yours it looks like it might have a little more crown in the deck in front of the dog house compared to a Spectra and some other differences but clearly it's a Spectra splash. A splash doesn't necessarily mean a direct copy, most everyone changed something. Tarva is a perfect example, lots of changes to their 2nd generation offering but it still at heart a Spectra.

    Last edited by old rigger; 03-17-2015 at 07:51 PM.

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    Senior Member jetboatperformance's Avatar
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    Forrest would recommend you try the boat without the wedges or droop (use a standard nozzle adapter) , zero out the tabs put the ride plate up a bit and although it looks set back a bit, try to get everything back to the basics and work from there Tom

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    Senior Member old rigger's Avatar
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    Quote Originally Posted by jetboatperformance View Post
    Forrest would recommend you try the boat without the wedges or droop (use a standard nozzle adapter) , zero out the tabs put the ride plate up a bit and although it looks set back a bit, try to get everything back to the basics and work from there Tom
    How in the hell are you supposed to zero out those tabs?

  16. #14
    Senior Member jetboatperformance's Avatar
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    Quote Originally Posted by old rigger View Post
    How in the hell are you supposed to zero out those tabs?
    Make them innefective altogether, raise them up out of the way ,or like Forrest said take them off temporarily

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